It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and "shelf-life" of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock. In the food packaging industry, several means for limiting oxygen exposure have already been developed. At present, the more commonly used means include modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging. In the first two instances, reduced oxygen environments are employed in the packaging, while in the latter instance, oxygen is physically prevented from entering the packaging environment.
Another, more recent, means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can provide a uniform scavenging effect throughout the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passing through the walls of the package (herein referred to as an "active oxygen barrier"), thereby maintaining the lowest possible oxygen level throughout the package.
One example of an oxygen scavenger incorporated into an oxygen scavenging wall is illustrated in European Applications 301,719 and 380,319 as well as in PCT 90/00578 and 90/00504. See also U.S. Pat. No. 5,021,515. The oxygen scavenger disclosed in these patent applications comprises polyamide/transition metal catalyst compositions. Through catalyzed scavenging by the polyamide, the package wall regulates the amount of oxygen which reaches the interior of the package (active oxygen barrier). However, it has been found that the onset of useful oxygen scavenging, i.e. up to about 5 cubic centimeters (cc) oxygen per square meter per day at ambient conditions, may not occur for as long as 30 days and, therefore, is not acceptable for many applications.
Further, in regards to the incorporation of the polyamide/catalyst system into the packaging material; polyamides are typically incompatible with the thermoplastic polymers, e.g., ethylene-vinyl acetate copolymers and low density polyethylenes, typically used to make flexible packaging materials and films. Even further, when polyamides are used by themselves to make a flexible package wall, they usually result in inappropriate stiff structures. Polyamides are also more difficult to process when compared with thermoplastic polymers typically used to make flexible packaging.
U.S. Pat. No. 5,399,289 discloses oxygen scavenger compositions composed of ethylenically unsaturated hydrocarbon polymers and transition metal catalysts. The polymers are required to have a low ethylenic double bond content of from 0.01 to 10 equivalents per 100 grams of polymer so as to provide a product with both scavenging properties and retained physical properties. Various conventional homopolymers, copolymers and polymer blends are disclosed. Because these polymers are amorphous materials they are difficult to blend and be processed with film forming semi-crystalline polymers, such as low density polyethylene and the like, which are conventionally used in providing flexible films and the like for packaging applications.
U.S. Pat. No. 5,211,875 also discloses the use of ethylenically unsaturated compounds in conjunction with a transition metal as well as a photoinitiator to facilitate initiation of the effective scavenging activity. The ethylenically unsaturated polymers and copolymers suggested by this reference are also amorphous materials and, therefore, have low compatibility with conventional film forming polymers, such as polyethylenes. Because of the limited compatibility of the scavenger polymer with the film forming polymer, one is required to limit the amount of scavenger polymer in the blend and is usually confronted with a resultant composition which is difficult to process.
It is highly desired to have an oxygen scavenger composition which is composed of a polymeric material having high processability which can be directly formed into films useful in the packaging field or have high compatibility with semi-crystalline polyolefins and provide a highly processable blend with such polymeric materials having known utility for packaging application.
Further, it is highly desired to have a film or composition composed of an ethylenically unsaturated polymer capable of scavenging oxygen which can substantially retain its physical properties after significant oxygen scavenging.
Still further, it is highly desired to provide an oxygen scavenger composition which does not provide, upon oxygen scavenging, by-product formation which can detract from the color, taste or odor of the packaged product.